1. Field of the Invention
This invention relates to the use of polyesters and/or polyamides containing sulfate and/or sulfonate groups as stabilizers for the emulsion polymerization of ethylenically unsaturated monomers, to a process for the production of stable polymer dispersions and to their use.
One of the particular advantages which emulsion polymerization is known to have over other processes, such as bulk or solution polymerization, is that, on the one hand, an emulsion-polymerized product is present in an ideal form (as a latex) for use in paints, coating compounds, adhesives and impregnating compounds and, on the other hand, emulsion polymerization is particularly suitable for simple process control.
In processes of the type in question, the choice of the emulsifier is a very important factor both in regard to process control and in regard to the properties of the subsequent latex. Thus, the emulsifier should form a stable emulsion between the monomer and water phases on the one hand and, subsequently, a stable latex on the other hand. Since the emulsifier remains in the end product, it should not impart any negative properties to the end product. For example, it is often undesirable if the emulsifiers used float to the surface during drying of the latex and lead to discoloration and/or inadequate water resistance of the film formed.
2. Discussion of Related Art
Numerous emulsifiers have been tested for their suitability for emulsion polymerization. Accordingly, it has long been known from the literature that emulsifiers and/or water-soluble polymers (so-called protective colloids) can be used as emulsifying agents in emulsion polymerization. A review of the most important and widely used emulsifying agents can be found in G. Schulz "Die Kunststoffe", C. Hanser Verlag, 1964, in Ullmann, Encyklopadie der technischen Chemie, 4th Edition, Vol. 10, Verlag Chemie, Weinheim (1980), pages 455-461 and in J. C. Johnson "Emulsifiers and Emulsifying Techniques", Noyes Data Corporation, Park Ridge, N.J. 1979.
In addition, it is known that fine-particle dispersions having particle sizes of 0.1 .mu.m to 0.3 .mu.m can generally be obtained by the use of emulsifiers or surfactants in emulsion polymerization while coarse-particle dispersions having particle sizes above 1 .mu.m can be obtained where protective colloids are used. It is generally assumed that the fine or coarse nature of the dispersions is the result of different reaction mechanisms. Polymerization in the presence of emulsifiers and/or surfactants takes place in micelles and leads to particles which are smaller than the originally dispersed monomer droplets whereas, in the presence of protective colloids, the reaction takes place in the monomer droplets so that the size of the droplets is responsible for the size of the polymer particles. The two reaction mechanisms can often be brought into place simultaneously by the combined use of emulsifiers and protective colloids or by the use of monomer-soluble radical initiators instead of water-soluble initiators. In this case, dispersions containing bimodal particles or at least dispersions having a very broad particle size distribution are generally formed. The average particle diameter is in the range from about 0.2 .mu.m to 2 .mu.m.
It is generally very difficult to produce emulsifier-free fine-particle polymer dispersions of high solids content. Processes for the production of stabilized polymer dispersions which enable fine-particle dispersions to be produced without the use of emulsifiers and/or surfactants have recently been disclosed in the patent and specialist literature. Although the reaction mechanism is not discussed in detail, the fine-particle emulsifier-free dispersions are said to have particular performance properties.
The stabilization of polymer dispersions with polyelectrolytes is described in DE 25 40 468. The dispersions are said to have high shear and salt stability, high pigment compatibility and only a slight tendency towards foaming. The polyelectrolytes used are water-soluble polymers containing recurring units of lateral carboxylic acid groups which are reacted off with monofunctional alcohols or primary amines which in turn bear a sulfonate function as an additional functional group. The chain is a carbon chain of the type formed, for example, by polymerization of (meth)acrylates. Although these polyelectrolytes have ester functions, they are not polyesters in the sense that the polymer chain is formed by ester units.
Polyesters and polyamides containing sulfonate groups are already known to the expert. Thus, according to DE 35 19 678 for example, polyesters containing sulfonate groups and alkyl side chains are used as dispersion aids for coal suspensions.
The polyesters in question are polyesters of carboxylic acid dialkanolamides and sulfodicarboxylic acids. These polyesters are soluble in water which is one of the requirements mentioned in the statement of problem in the above-cited patent. In addition, the fact they are suitable as dispersants for coal suspensions does not mean that they are also suitable as stabilizers for emulsion polymerization because emulsion polymerization stabilizers have to meet different and, in some cases, additional requirements, for example they must not adversely affect the performance properties of the polymer dispersion produced.
The problem addressed by the present invention was to provide stabilizers suitable for the emulsion polymerization of ethylenically unsaturated monomers which would lead to fine-particle polymer dispersions showing improved resistance to water and producing a high level of gloss in the films produced from them. In addition, there would be no need, to use emulsifiers or surfactants in this regard.